Some polyamides are used in the packaging industry because of their clarity, thermoformability, high strength and toughness over a broad temperature range, chemical resistance and barrier properties against gases, oils, fats and aromas. They are generally used as the barrier component in rigid packaging applications and in multi-layer structures. Polyamides or nylons, as they are also called, are thermoplastics characterized by the presence of repeating amide groups. The various types of nylons differ according to the structure of the organic segments separating the amide groups.
Film-forming nylons are usually linear and conform to one of two general structures: ##STR3##
Polyamides of the first type are made from amino acids or their lactams; examples are nylon-6, nylon-11, and nylon-12, wherein the numbers correspond to the total number of carbon atoms in the repeat units. The second type of polyamides are made from diamines with dicarboxylic acids or their derivatives; examples are nylon-66 and nylon-610, wherein the first number corresponds to R' and refers to the number of methylene groups between the nitrogen atoms, and the second number corresponds to the number of carbon atoms in the COR"CO groups. Nylon copolymers can also be prepared from mixtures of diamines and dicarboxylic acids or their derivatives.
Various nylon processing methods and packaging applications are described in Tubridy et al., M. Baker, ed., The Wiley Encyclopedia of Packaging Technology, pp. 477-482 (Jon Wiley & Sons, NY 1986).
Barrier resins, i.e. materials which impede the permeation of small molecules through the polymer structure, have revolutionized the packaging industry in recent years. The oxygen permeability of some barrier polymers increases appreciably with increasing relative humidity. Examples are polyvinyl alcohol, ethylene-vinyl alcohol copolymers, and nylons 6 and 66. Nylon-66, for example, has an oxygen permeability at 30.degree. C. and 0% relative humidity of 2.4.times.10.sup.-12 cc-cm/cm.sup.2 -sec-cm Hg, but at 80% relative humidity its oxygen permeability is 4.9.times.10.sup.-12 cc-cm/cm.sup.2 -sec-cm Hg and continues to increase rapidly as the relative humidity approaches 100%. Poly[hexamethyleneisophthalamide/terephthalamide], an amorphous nylon barrier resin available under the trade designation SELAR.RTM. PA is an exception; at 30.degree. C., the oxygen permeability of SELAR.RTM. PA films at 80% relative humidity is reported to be less than it is at 0% relative humidity. See Krizan et al., Polymer Preprints (ACS), pp. 9-10 (Spring 1989).
The preparation of polyamides is well known, and is described, for example, in Morgan, Condensation Polymers: By Interfacial and Solution Methods, pp. 163-260 (Interscience Publishers, 1985). The preparation of regular aliphatic copolyoxamides is described by Chang et al., in Journal of Polymer Science: Polymer Chemistry Edition, vol. 15, pp. 1043-1060 (1977). The preparation of regular aliphatic/aromatic copolyoxamides is described in Stevenson et al., Journal of Macromolecular Science-Chemistry, A11(4), pp. 779-809 (1977).
From U.S. Pat. No. 3,935,172 to Vogl et al., it is known to prepare reverse osmosis membranes from regular amide/oxamide copolymers by casting a film of the polymer from a solution, partially evaporating the solvent, and fixing the polymer by gelation with a suitable non-solvent. These perm-selective membranes are stated to be useful in the desalination of water by reverse osmosis.
From U.S. Pat. Nos. 4,085,163; 4,111,896; and 4,126,600, all to Gergen et al., it is known to include a polyamide in a blend composition with an arene-diene block copolymer and a dissimilar engineering thermoplastic resin, such that at least two of the polymers form at least partial continuous network phases which interlock with the other polymer networks.
From U.S. Pat. No. 4,119,615 to Schulze, it is known to prepare a thermoplastic adhesive composition based on a polyoxamide prepared by reacting a polyoxypropylene polyamine with oxalic acid.
Other composite structures such as laminates which contain a polyamide or similar material are well known in the art. Representative of such art are U.S. Pat. Nos. 3,514,367 to James; 3,570,748 to Coyle et al.; 3,733,214 to Stockhausen; 3,890,448 to Ito; 4,058,647 to Inoue et al.; 4,085,244 to Stillman; 4,104,438 to Angelo et al.; 4,105,818 to Scholle; 4,182,457 to Yamada et al.; 4,296,156 to Lustig et al.; 4,309,466 to Stillman; 4,405,667 to Christensen et al.; 4,410,482 to Subramanian; 4,416,942 to DiLuccio; 4,444,817 to Subramanian; 4,559,266 to Misasa et al.; 4,579,773; to Cole et al.; 4,596,866 to Jackson, Jr. et al.; 4,612,221 to Biel et al.; and 4,842,946 to Foust et al.